The MOZ_MUST_USE macro is defined as clang's and gcc's nonstandard __attribute__((warn_unused_result)). Now that we compile as C++17 by default (bug 1560664), we can replace MOZ_MUST_USE with C++17's standard [[nodiscard]] attribute.

BitsUtils.cpp only references MOZ_MUST_USE in a comment about forking a copy of mozilla::ScopeExit to remove some dependencies on other Mozilla header files (including MOZ_MUST_USE from mfbt/Attributes.h). [[nodiscard]] doesn't require a header file, so we can just remove this comment about MOZ_MUST_USE.

Differential Revision: https://phabricator.services.mozilla.com/D89296